Since my luggage has been known to go astray--although not recently, thank goodness!--I pack the things I absolutey have to have for 2 days in my carry-on!
Change of clothing (sometimes add a thermal knit top for winter)
Two sets of underwear
Sleepwear
Jacket (usually visit WDW in December and I am known for losing jackets!)
Pair of Birks (wear my sneakers)
Eyeglasses
Extra pair of contact lenses
Hairbrush/Comb
Toothbrush
Laptop with geek bag
OTC meds in pill-form (antihistimine, pain relief, etc.)
3-1-1 bag (includes travel size contact lens solutions)
Duplicate set of travel documents
Extra reading material (five-hour flight + airport time)
I also carry a mid-sized purse (always! *LOL*), so my cell phone, camera and original travel documents are in that.
To date, my luggage always has been returned to me within about 48 hours. That's why I pack stuff for two days! *LOL*
Edited to add:
Since my carry-on bag is stowed in the overhead compartment, I always lock it just before boarding.
